1. Field of the Invention
The present invention relates to a method and test kit for determining the tendency of a crude oil to foul oil field refinery equipment, and more particularly, to a method and test kit for optimizing the dosage of antifouling additives used in treating a given sample of crude oil.
2. Description of the Prior Art
Fouling is a common problem in the refining of crude oil. Fouling is defined for purposes of the present invention as the deposition of inorganic and carbonaceous substances on heated oil refinery surfaces by crude oil. These deposits reduce the rate of heat transfer to the crude oil, and eventually, reduce throughput rates. In fact, these deposits can, in some instances, block the flow of crude oil through the equipment. Fouling results in increased energy costs, increased maintenance costs, and capital expenditures for the modification or replacement of refinery equipment, e.g., heat exchangers.
The mechanism of fouling is not fully known. However, some investigators have suggested that several different components of crude oil may contribute to fouling; i.e., asphaltenes, coke, organic polymers and organic reaction products, inorganic silicates, inorganic salts, metal oxides or sulfides. Because fouling severely reduces efficiency in the refining of crude oil, there are a number of methods and devices which are designed to predict the fouling tendency of a particular crude oil.
One method which predicts the fouling tendency of a crude oil measures the rate of heat transfer from a heated refinery surface to the crude oil. The rate of heat transfer is inversely proportional to the deposition of fouling materials onto the heated refinery surface. For example, the crude oil temperature at the exit of a heat-exchanger is measured while the metal temperature of the heat-exchanger is controlled. As the fouling progresses, deposits build up on the heated surfaces of the heat exchanger. Significant fouling is indicated by a reduction of heat transfer, i.e., a decrease in the crude oil outlet temperature. The change in crude oil outlet temperature with time provides the basic heat data required for the comparative evaluation of different crude oils. This data is used to extrapolate the relative fouling tendency of a given crude oil.
This method is used in the laboratory in accelerated tests which are designed to reproduce the fouling problem experienced in a refinery over several months. Fouling acceleration is provided by carrying out tests at operating temperatures higher than those in operating refinery units. With the rate of fouling increased, a crude oil's fouling characteristics can be assessed in a shortened period of time, i.e., from about 3 to about 4 hours. Each test unit is capable of performing about 200 tests annually.
The above method is useful to determine the general propensity of a crude oil to foul equipment. Recently, however, one component of crude oil, asphaltenes, has been scrutinized as contributing heavily to the fouling problem. Therefore, methods and devices have been developed to help determine fouling profiles of crude oils containing asphaltenes. For example, U.S. Pat. Nos. 4,781,893, and 4,781,892 Dickakian, describe an apparatus and method for detecting the presence of incompatible asphaltenes in a given crude oil sample, and estimating the tendency of the crude oil to foul equipment. According to the patents, incompatible asphaltenes foul equipment by separating from the crude oil and adhering to a heated metal surface where they are changed into coke-like material. It is this coke-like material which fouls the heated surface. According to the method described in the referenced patents, a single drop of crude oil is deposited on a polymeric membrane constructed from polymers containing polar atoms. The sample migrates on the membrane to form concentric rings. The concentric rings consist of hydrocarbons separated by size and incompatible asphaltenes. Light reflected from the surface of the sample is processed to determine the constituents of each concentric ring. In this way the presence of incompatible asphaltenes is determined and the fouling tendency of oil is estimated.
Once the fouling tendency of a crude oil is estimated, the dosage of the antifouling agent is estimated. Antifouling agents are added to the crude oil during the refining process. Antifouling agents are utilized to help alleviate the problem of fouling. If the crude oil contains asphaltenes, asphaltene dispersants are typically added to the crude oil. These asphaltene dispersants suspend or disperse the agglomerated or precipitated asphaltenes in the crude oil. Therefore, asphaltenes do not precipitate from the crude oil onto the heated metal refinery surfaces where they foul the equipment.
Presently, the dosage of antifouling agent added to the crude oil is estimated by first estimating the fouling propensity of the crude oil. The greater the likelihood that the crude oil will foul equipment, the larger the dose of the antifouling agent added. However, estimating the dose of the antifouling agent is an inefficient and commercially unacceptable method for controlling the refining of crude oil.
The problem with estimating dosages is that if the dosage of the antifouling agent added to the crude oil is too low, throughput rates are lowered and equipment is damaged. Further, if the estimated dosage is too high, antifoulant is wasted, and accordingly, money is spent ineffectively. It becomes apparent when considering the world-wide need for oil and the limited resources available to develop it, that any vehicle which creates efficiencies in the refining of oil is potentially important commercially and socially. Accordingly, the refining industry requires more effective dosage control of the various chemical additives in their refineries. The purpose of dosage control devices or systems is to optimize on a more frequent basis the dosage required for a given refining process. In the past, as discussed above, optimization programs have proven to be minimal.
Although there are methods and devices available for estimating the fouling tendency of a crude oil containing asphaltenes, the results yielded are not specific enough to enable a unit operator to accurately and efficiently control asphaltene fouling during the refining process. Further, these estimation methods are time consuming and expensive. For example, estimating the fouling tendency of a crude oil based on the rate of heat exchanged over a period of time is an expensive and time consuming process. This procedure requires a specially constructed heat-exchanging unit which must be operated in a laboratory. The process requires three to four hours to complete for each sample, and the units must be thoroughly cleaned between trials. Furthermore, each unit typically only evaluates about 200 samples annually. Thus, in order to complete a comparative evaluation study of antifoulant treated crude oil and untreated crude oil, several days, if not weeks, would be required. This reduces the applicability of the process to long-term planning, and accordingly, the process is relatively useless in the short-term efficient refining of crude oil.
The methods and devices described in the 4,781,893 and 4,781,892 patents require from several minutes to several hours for the development of the measurable chromatographic pattern. These methods are directed to determining the relative presence of asphaltenes in a crude oil. From this data the relative fouling tendency of a given crude oil sample is estimated. These methods and apparatus are not directed to optimizing the dosage of a particular antifouling agent in a given crude oil sample. In fact, these references even fail to identify the accurate dosing of antifouling agents as critical to the efficient refining of crude oil. Thus, these methods are not applicable to the on-site "fine tuning" of the refining process.
Hence, in light of the deficiencies in the art set forth above, it would be advantageous to provide a fast, accurate, simple, reproducible, and inexpensive method for optimizing the dosage of an antifouling agent necessary to add to a given sample of crude oil to alleviate asphaltene fouling of oil refinery equipment. Further, it would be even more advantageous to provide a method which could be employed on-site in the refinery by unit operators without extensive chemical training to optimize the dosage of a particular antifouling agent that when added to the crude oil would alleviate asphaltene equipment fouling. Therefore, it would be particularly advantageous to provide a test kit for use by unit operators which would not only monitor the asphaltene fouling tendency of a crude oil but would also optimize the dose of an antifouling agent necessary to add to a crude oil to alleviate asphaltene fouling.